Field of the Invention
The invention relates to a fan blade structure and, more particularly, to a fan blade structure applied to a centrifugal fan.
Description of the Related Art
Most electronic devices, such as a notebook computer and a projecting device, generate much heat while operation. A heat dissipating device, such as a fan, is usually disposed inside the electronic device to remove heat from the electronic components to extend service life. Moreover, since the electronic devices become thinner, most electronic devices have less and less space for natural convection in casings. Thus, the thin electronic device usually uses a centrifugal fan to develop a forced convection inside the casting.
FIG. 1 is a schematic diagram showing a conventional centrifugal fan. The centrifugal fan 1 includes at least a casing 11 and a fan blade structure 12. The casing 11 forms an accommodating space and an air outlet 110. The fan blade structure 12 is disposed in the accommodating space. The fan blade structure 12 includes a plurality of blades 121 which draws air in and blows airflow out from the air outlet 110.
When the blades 121 rotate, the wake flow generated by the blades 121 hits the surface of a tongue structure 112 at the bottom of the casing 11. Each hitting point can be taken as a sound source of the narrow band noise. The wake flow would generate the blade passing tone noise which has large amplitude and a constant frequency according to a wave superposition principle. In other words, when a rotating speed of the conventional centrifugal fan increases, the level of the noise also increases.